1. Field of the Invention
This invention relates to heavy duty pneumatic tires used for truck and bus, and more particularly to a heavy duty pneumatic tire capable of effectively preventing the heat accumulation in a shoulder region of a tread portion to sufficiently improve the wear resistance.
2. Related Art Statement
In this type of tire, it has hitherto been attempted to change the compounding of the tread rubber to improve the durability of the tire. According to this technique, the hysteresis loss becomes larger as the wear resistance is improved. Hence the heat build-up becomes large, and particularly separation or other failure is apt to be caused in the shoulder portion due to heat accumulation in the tread shoulder portion having a thicker rubber thickness. Therefore, in order to eliminate such a problem for the improvement of the durability, it is recently and widely practiced that the tread rubber comprises two inner and outer rubber layers b, c having a cap and base structure as shown in FIGS. 3a and 3b. A low heat build-up rubber is used as the base portion b.
In the conventional technique, the cap portion c and the base portion b are separately formed due to the two layer structure of the tread rubber, so that it is necessary to accurately place these portions on a given position and consequently the operability is poor. This is also a fear of air remaining between both the portions. On the other hand, a dual extruder is required for simultaneously conducting the formation of cap portion c and base portion b and always conducting the adequate formation thereof, resulting in considerable increase of equipment cost.
Furthermore, the base portion b composed of low heat build-up rubber has less bond layers between rubber and carbon, so that cracks are apt to be grown and also the cut resistance is poor. Consequently the appearance at last worn stage becomes poor.